The long-term objectives of this proposal are to elucidate the mechanisms by which Plasmodium sporozoites localize to and attach to their target cells in both mammalian and mosquito hosts. The major sporozoite surface protein, the circumsporozoite protein (CS, participates in target cell recognition in both hosts. Previous work has shown that CS binds to hepatic heparan sulfate proteoglycans (HSPGs) and that this binding functions during hepatocyte invasion. To date, the molecular events involved in salivary gland recognition have not been characterize, although we have evidence that CS participates in this process. The specific aims of this process are: 1) To define the structural requirements of HSPGs required for CS binding by affinity purification and characterization of region II-plus binding glycosaminoglycan (GAG) chains. In addition, using newly developed assays that enable us to follow sporozoites from the circulation. 2) To determine the structural properties of CS responsible for binding to mosquito salivary glands by using synthetic peptides as competitive inhibitors of CS binding to glands and to determine whether differences in the salivary gland binding region of CS is responsible for the transmission of the avian malaria parasite P. gallinaceum by Culicine rather than Anophelene mosquitos. 3) By expression cloning, to isolate and characterize the salivary gland receptor for CS. The work we propose is of relevance to the development of new drug therapies for malaria and the current interest in the use of transgenic mosquitos in vector control programs. For example, knowledge of the structural properties of HSPG GAG chains which bind to CS may lead to the development of drugs which could inhibit sporozoite localization to the liver. In addition, knowledge of the structural properties of receptors and ligands involved in salivary gland invasion could be used to generate transgenic mosquitos refractory to infection with Plasmodium.